(1) Field of the Invention
The present invention generally relates to a coding system compressing video data and sending compressed video data to a transmission channel, and more particularly to such a coding system having a data controlling mechanism which is activated when an error is detected.
In a video conference system or a video phone system, it is required that the quantity of information to be transmitted be reduced. Normally, an interframe predictive coding procedure is used to compress video data to be transmitted. When a system as described above is activated, or when a scene changes, video data is compressed and transmitted by an intraframe coding procedure. This intraframe coding procedure is also activated when the coding system is notified of the occurrence of error.
(2) Description of the Prior Art
Referring to FIG. 1, a conference room (terminal) A and a conference room (terminal) B are coupled with each other via a communication channel, and communicate with each other. Information composed of video data and speech data is transmitted from the terminal A to the terminal B, so that a speaker and a display device of the terminal B are driven by the received information. Meanwhile, information composed of video data and speech data is transmitted from the terminal B to the terminal A, so that a speaker and a display device of the terminal A are driven.
If an error ERR occurs in the transmission channel from the terminal A to the terminal B, the terminal B detects the error ERR, and sends a coding mode change request signal REQ to the terminal A. This coding mode change request signal REQ is superimposed on information to be sent to the terminal A. Upon receiving the coding mode change request signal REQ, the terminal A sends intraframe-coded data D2 to the terminal B.
FIG. 2 shows a transmit/receive part of each of the terminals A and B. A video signal S1 input via a preprocessing circuit (which digitizes an analog video signal from a video camera into a digital video signal for every block) is input to an interframe coding unit 11 and an intraframe coding unit 12, both of which code the input video signal S1 by the respective coding procedures. A switch SW is provided on the output sides of the interframe coding unit 11 and the intraframe coding unit 12. Normally, the switch SW is switched to the interframe coding unit 11. Thus, video data coded by the interframe coding frame 11 passes through the switch SW and is supplied to a multiplexer 13, which multiplexes the coded video data with control data, such as auxiliary information about coefficient data with a run-length coding used. Multiplexed data generated and output by the multiplexer 13 is buffered in a buffer memory 14, and then sent to the other party's terminal via the transmission channel.
Meanwhile, multiplexed data $2 sent from the other party's terminal via the transmission channel is buffered in a buffer memory 15, and then applied to a demultiplexer 16, which separates the superimposed control data from the received multiplexed data S2. The video data and the control data are separately supplied to a decoding unit 18, which decodes the separated video data to thereby reproduce the original video data. The control data is also supplied to a coding mode controller 17. The reproduced video data is supplied to a display device and a speaker (not shown).
When the error ERR occurs and the terminal B detects the error ERR, it superimposes the coding mode change request signal REQ on part of the control data, which is normally composed of a plurality of bits. That is, the multiplexed data S2 includes the coding mode change request which is part of the control data and which is represented by one or more bits. The separated coding mode change request signal REQ is input to the coding mode controller 17, which waits for the completion of coding a frame (which corresponds to one picture plane) being processed. When it is recognized that the coding of the frame being processed is completed, the coding mode controller 17 instructs the switch SW to select the intraframe coding unit 12. Thus, the multiplexer 13 receives intraframe-coded data regarding the next frame via the switch SW, and multiplexes the received intraframe-coded data with the control data.
The interframe-coded data is obtained by encoding a difference between the current frame and the immediately previous frame, and it is thus impossible to reproduce the original picture plane (video signal) from a piece of interframe-coded data itself. Meanwhile, the intraframe-coded data is obtained by encoding data within a frame, so it is possible to reproduce the original video signal. Thus, even when an error takes place during communication, the original picture plane can be reproduced from the intraframe-coded data instead of the interframe-coded data.
One picture place consists of, for example, 525 scanning lines, and the video signal of each scanning line is digitized. During the coding procedure, one block of data amounting to 8.times.8 pixels is defined, and the video signal is digitized for every 6 blocks. The signal transmitted between the terminals A and B has a format in which one frame forms one picture plane. More specifically, the format is defined in, for example, the CCITT Recommendation H.261. According to this recommendation, each frame corresponding to one picture plane has a picture start code (PSC) at the beginning thereof, subsequent control bits and video data.
When the coding mode change request REQ is received, the switch SW is switched to the intraframe coding unit 12 from the interframe coding unit 11. At this time, normally, the interframe-coded data amounting to one or two frames remains in the buffer memory 14. Thus, the intraframe-coded data cannot be sent to the destination terminal until the interframe-coded data is completely read out from the buffer memory 14. Further, although not shown, the multiplexer 13 has internal buffers. Thus, it is necessary for data in the internal buffers to be completely read out therefrom before the intraframe-coded data is sent to the destination terminal. Moreover, the switch SW is switched to the intraframe coding unit 12 after the interframe coding procedure on the frame being processed is completed, so that the interframe-coded data is continuously output to the destination terminal before the switch SW is switched, although the destination terminal has already sent the coding mode change request signal REQ to the source terminal. Thus, it is impossible to send the intraframe-coded data to the destination terminal immediately after the switch SW is switched, so it takes a long time for the destination terminal to receive the intraframe-coded data and reproduce the normal picture planes therefrom.